Porous sheets for use as air permeable bags, pressure filtration membranes, filtration supports and so forth have been produced by the following methods:
(1) a method as described in Japanese Patent Application (OPI) No. 74667/75 (the term "OPI" as used herein refers to a "published unexamined Japanese Patent application") in which an inorganic filler capable of being eluted by acids, alkalis or water and a nonionic surface active agent having HLB (Hydrophilic-Lipophilic Balance) of 9 to 15 are added to a polyolefin resin, the resulting mixture is molded into a sheet and the sheet is stretched, and then the above inorganic filler is eluted by an acid or the like as described above to produce a porous sheet;
(2) a method as described in Japanese Patent Application (OPI) No. 43982/79 in which in the method as described in (1) above, an ethylene-vinyl acetate copolymer is used in place of the nonionic surface active agent having HLB of 9 to 15 to produce a porous sheet;
(3) a method as described in Japanese Patent Application (OPI) No. 47334/82 in which a filler and a liquid rubber are compounded to a polyolefin resin, the resulting composition is melt molded to obtain a sheet, and then the sheet is stretched to produce a porous sheet;
(4) a method as described in Japanese Patent Application (OPI) No. 20352/82 in which in the method as described in (3) above, polyhydroxy saturated hydrocarbon is used in place of the liquid rubber to produce a porous sheet; and
(5) a method as described in Japanese Patent Application (OPI) No. 136334/84 in which a polyolefin resin selected from specified intermediate pressure and low pressure polyethylenes, and crystalline polypropylene, a filler, a polyhydroxy saturated hydrocarbon and an epoxy group-containing organic compound are compounded, the resulting composition is melt molded into a sheet, and then the sheet thus obtained is stretched to produce a porous sheet.
In accordance with the above methods, the objective sheet is directly obtained by stretching, or alternatively the porous sheet obtained by stretching is treated with an acid, for example, to elute the inorganic filler contained in the porous sheet, thereby obtaining the objective porous sheet having more increased permeability. This stretching causes orientation of molecules, thereby increasing the strength of the sheet. In this stretching, however, perforated holes are formed and the orientation of molecules becomes uneven and, furthermore, at the time of stretching, fine cracks are formed in the perforated holes of the sheet. This phenomenon occurs more frequently when the compatibility between the resin and the filler is poor, or the amount of the filler added is increased, or the stretching ratio is increased. As a result, disadvantages such as a decrease of the tensile strength in the stretched direction, a decrease of yield stress and a decrease of mechanical strength, are caused.
In order to overcome the above problems, a method has been proposed in which a porous substrate such as nonwoven, cloth, and porous plastic sheet, is used as a reinforcing member and laminated onto the porous sheet made of a synthetic resin.
In accordance with the above lamination method, a bonding agent or an adhesive is partially coated on a porous substrate and/or a plastic porous sheet, and at these portions, they are laminated.
The laminate obtained by partially bonding the porous substrate and the porous sheet using the adhesive, however, has disadvantages such that the bonding strength of the bonding agent is poor and layer separation sometimes occurs during the process of molding or in use, no sufficient reinforcing effect can be obtained, and reliability is poor.
On the other hand, in partially bonding the porous substrate and the porous sheet with a hot melt type adhesive, the problems as described above can be eliminated. In a case where the hot melt type adhesive is used, areas where the adhesive is coated are heated by the means of a heating member such as an embossing roll. Thus, it is necessary for the heating member to be controlled so as to be accurately brought into contact with the areas being heated. This produces the disadvantages that the production equipment becomes complicated and expensive, and the process of production of the sheet becomes complicated, thereby increasing the production costs.
As a result of extensive investigations, it has been found that if a plastic porous sheet is used as the porous sheet, a hot melt type porous sheet is sandwiched between the plastic porous sheet and a porous substrate, and the resulting laminate is heated and bonded, mechanical strength such as tensile strength or yield stress of the sheet is greatly increased.